1. Field of the Invention
The present invention relates to a wideband active balance-to-unbalance (balun) circuit based on a differential amplifier and, more specifically, to a wideband balun circuit used for a radio-frequency (RF) transceiving system, which converts a single-ended RF input signal into two complementary differential output signals.
This work was supported by the IT R&D program of Ministry of Information and Communication/Institute for Information Technology Advancement [2006-S-015-01, Development of Digital RF and ADC Chips for Multi-Mode SDR Terminal.]
2. Discussion of Related Art
A wireless transceiving system wirelessly transmits and receives information through air from a distance. In order to ensure the quality and reliability of the transmitted information, the wireless transceiver performs a modulation operation in which an RF local oscillation frequency carries a signal, and a demodulation operation in which the local oscillation frequency is removed from a received signal to reproduce an original signal.
A frequency converter, which performs the above-described important modulation and demodulation operations, is the essential component that determines the communication quality of the wireless transceiving system. FIG. 1 illustrates an RF front-end portion of a receiving system including a frequency converter.
Referring to FIG. 1, the RF front-end portion of the receiving system includes a low-noise amplifier (LNA) 110, which low-noise amplifies a received RF signal RF, an RF balun circuit 120, which outputs the amplified RF signal RF as a differential signal, a local oscillation signal generation circuit 130, which generates a local oscillation signal LO, a double balance mixer 140, which converts an RF signal into an intermediate frequency (IF) signal IF, and an IF output circuit 150, which amplifies the IF signal and outputs the amplified IF signal.
The balun circuit 120 is used to divide a single input signal into two phase-inverted output signals. In order to obtain phase-inverted complementary signals having the same amplitude, a balanced structure is typically used to convert a single-ended input signal into a complementary differential output signal.
The construction of a conventional active balun circuit 120 based on a differential amplifier will now be described with reference to FIG. 2.
Referring to FIG. 2, a first resistor R21 is connected between a power supply voltage terminal VDD and a first node Q21, and a second resistor R22 is connected between the power supply voltage terminal VDD and a second node Q22. Drain and source terminals of a first NMOS transistor N21 are connected between the first node Q21 and a third node Q23, and drain and source terminals of a second NMOS transistor N22 are connected between the second node Q22 and the third node Q23. A third NMOS transistor N23 having a gate terminal to which a predetermined bias voltage is applied by a bias terminal BIAS is connected between the third node Q23 and a ground terminal GND. A first capacitor C21 is connected between an input terminal VIN and a gate terminal of the first NMOS transistor N21. A third resistor R23 is connected between a bias terminal VB for applying a specific bias voltage and the gate terminal of the first NMOS transistor N21, and a fourth resistor R24 is connected between the bias terminal VB and a gate terminal of the second NMOS transistor N22. A second capacitor C22 is connected between the first node Q21 and a first output terminal VO1, and a third capacitor C23 is connected between the second node Q22 and a second output terminal VO2. A first inductor L21, a fifth capacitor C24, and a fifth resistor R25 are connected in series between the first node Q21 and the gate terminal of the second NMOS transistor N22.
In the above-described construction, one terminal of the active balun circuit 120 based on the differential amplifier is AC grounded, while a single-ended input signal is applied to the other terminal of the active balun circuit 120 to obtain differential output signals. In this case, the differential output signals become unbalanced due to a capacitive loading effect of a parasitic capacitor caused by the third NMOS transistor N23 functioning as a tail current source. In particular, when an RF signal is applied to the active balun circuit 120, unbalance between the two differential output signals becomes more serious.
In order to overcome this drawback, U.S. Patent Registration No. 6,121,809 entitled “Accurate and Tuneable Active Differential Phase Splitters in RFIC Wireless Applications” has been disclosed. In this disclosure, a feedback circuit is structured by a first inductor L21, a fifth capacitor C25, and a fifth resistor R25 and inserted between a first node Q21 and a gate node of a second NMOS transistor N22 to compensate for signal unbalance.
According to the above-mentioned U.S. Patent Registration No. 6,121,809, signals are balanced by feeding a low-amplitude input signal back to a node from which a high-amplitude signal is output. However, it is difficult to apply this technique to a wideband active balun circuit, and a chip area is increased due to a bulky inductor.